This invention relates to a resonant sensing device used for measuring material properties as disclosed in U.S. Pat. No. 3,153,338 issued to C. Kleesattel on Oct. 20, 1964 entitled "Resonant Sensing Devices". The disclosed device comprises an elongate transducer having a workpiece engaging tip at one end, means for rendering the transducer resonant at its natural frequency of oscillation, and means for measuring and indicating the frequency shift of the transducer from its free resonant condition to the condition when the workpiece engaging tip is urged with predetermined static force into contact with the surface of a workpiece to be measured. The impedance of the workpiece, being related to the modulus of elasticity of the workpiece, loads the transducer during such contact and causes the resonant frequency of the transducer to shift to a higher value. This shift is a measure of the hardness of the workpiece and can be expressed, for instance, in units of Rockwell or Vickers.
A typical circuit for determining the frequency shift of the transducer from its unloaded to its loaded condition has been disclosed in U.S. Pat. No. 3,323,352 issued to Norman G. Branson on June 6, 1967 entitled "Control Circuit for Resonant Sensing Device". In this circuit the frequency shift of the transducer is converted by a frequency discriminator to an electrical direct current voltage signal whose amplitude is indicated by the deflection of a pointer on a conventional current measuring instrument. Using this type of measuring circuit two adjustable means are necessary for adjusting respectively the zero point setting and the scaling.
Due to the relatively small value of the frequency shift which is to be determined, very exacting requirements with respect to the stability of the discriminating circuit and the resonant frequency range, must be met. In practice these requirements are difficult to fulfill. Therefore, calibration measurements using calibrated test specimens are required repeatedly during the measurement process, specifically to establish the zero point, see C. Kleesattel and G. M. L. Gladwell, "The Contact Impedance Meter", Ultrasonics (magazine) Illiffe Publications, London, England, July 1968, October 1968, January 1969, and January 1970.